This invention relates to a band rejection filter, utilizing optical principles, which is employed in a diplexer or filter in the millimeterwave and submillimeter-wave transmission.
It is known in the art that a circuit of a small loss can be obtained by utilizing optical principles for electromagnetic waves of short wavelengths, and this technique has already been employed in diplexers for the millimeterwave transmission, for example, in a Michelson-interferometer type band-splitting filter, a filter using dielectric disks for suppression of the leakage of transmitter signals into receiving paths. However, these circuits have the defects of bulkiness and complexity in construction. That is, for the purpose of splitting frequency bands, the band-splitting filter employs a cutoff filter which permits the passage therethrough of electromagnetic waves of a frequency above a predetermined value, so that a tapered waveguide as long as 200 to 500 mm is required for the connection of the cutoff filter with an oversized waveguide. Accordingly, this conventional diplexer is very bulky.
A band rejection filter is usually employed as a filter for suppression of the leakage of transmitter signals into receiving paths, but since no quasioptical band rejection filter has been obtainable with the prior art, a low-pass filter is used instead. In the millimeterwave transmission system, however, use is made of a low-pass filter of the type which does not utilize the frequency band, for instance, below 40 GHz but uses the frequency band below 70 GHz as a pass band. As a result of this, the pass band becomes unnecessarily wide and, at the same time, the cutoff response of the frequency response is degraded, so that it is necessary to increase the number of cavities forming the low-pass filter. This inevitably increases the accuracy of fabrication and the cost of the filter and makes the filter bulky.
Further, there has been proposed a band-splitting filter employing two 3 dB hybrids and two quasioptical low-pass filters of the same characteristic. Since the frequency characteristic of this filter has ripples in the pass bands, and consequently the cutoff response is not so sharp, the number of cavities making up each low-pass filter must be increased, which makes the filter large and raises the accuracy of its fabrication and its cost.
Also in the fields of radio astronomy and plasma diagnostics of the millimeterwave and submillimeterwave bands other than the millimeterwave transmission, a diplexer and a filter are required. But the conventional diplexer and filter have also the same defects as mentioned above.
An object of this invention is to provide a band rejection filter utilizing optical principles.
Another object of this invention is to provide a quasioptical band rejection filter which is relatively simple in construction and sharp in cutoff response.
Another object of this invention is to provide a quasioptical band rejection filter which is easy to manufacture and adjust.
Still another object of this invention is to provide a quasioptical band rejection filter which enables easy construction of a diplexer or a filter for suppression of the leakage of transmitter signals into receiving paths.